A biomimetic sensor for detecting the presence of airborne toxins, such as carbon monoxide ("CO"), mercury, ethylene oxide, volatile organic materials, and hydrogen sulfide is disclosed in U.S. Pat. No. 5,063,164, the contents of which are hereby incorporated by reference. The sensor comprises a self-regenerating chemical sensor reagent impregnated into a substrate. The substrate is made of a porous solid state material which is sufficiently transmissive to light to permit detection of optical characteristics of the sensor using an optically coupled light emitting diode and photodiode.
Operating in clean air, the sensor is in an equilibrium condition indicated by a translucent yellow/orange color displayed on the sensor. If an airborne toxin such as CO is present, the sensor equilibrium is shifted as the reagent undergoes changes in its optical density, and the sensor begins to darken. After a time, which depends upon the CO concentration and the duration of exposure to CO, the sensor reaches a darkened condition that is predetermined to be an alarm point. When clear air is restored, the sensor eventually lightens to its initial orange color as it returns to the equilibrium condition.
The alarm point is a fixed point. This type of fixed alarm point, while effective, suffers from certain problems that may affect the performance of the sensor. First, even low levels of CO cause the sensor to darken appreciably if the low level condition is maintained over a long enough time. Therefore, an alarm can occur even though the level of CO may be well below a dangerous level. Even if the alarm is not triggered, the partially darkened sensor becomes more sensitive to CO and the alarm becomes susceptible to sounding based on much lower levels of CO than desired.
Second, when CO is present in sufficient concentrations and for a long enough time, the sensor darkens to the alarm condition. Depending upon the dose of CO to which the sensor is subjected, when placed in clean air, the sensor will remain dark for a considerable length of time as the sensor reagent returns to its equilibrium position. In this case, the alarm may remain energized long after a clear air condition has been established.
One skilled in the art would appreciate an apparatus and method for enhancing the response of existing biomimetic sensors. Such an apparatus and method would increase the desirability of such sensors, and make them more sensitive to actual conditions of airborne toxins.